Hello cuong.nguyenquoc,
The buffer size is the amount of time in milliseconds of port bandwidth that a queue can use to continue to transmit packets during periods of congestion, before the buffer runs out and packets begin to drop.
On EX Series switches except EX4300 switches, the default scheduler transmission rate and buffer size percentages for queues 0 through 7 are 95, 0, 0, 0, 0, 0, 0, and 5 percent, respectively. On EX4300 switches, the default scheduler transmission rate and buffer size for queues 0 through 11 are 75, 0, 0, 5, 0, 0, 0, 0, 15, 0, 0 and 5 percent, respectively, of the total available buffer.
To deduct packet drop on a specific queue, you can check which queue is getting packet bursts and dropping more packets. Depending on that, you can specify the buffer-size in the scheduler configuration.
Use a scheduler map to map the scheduler to the forwarding class that is mapped to the queue to which you want to apply the buffer size.
For example, suppose that you want to change the dedicated buffer allocation for FCoE traffic. FCoE traffic is mapped to the fcoe forwarding class, and the fcoe forwarding class is mapped to queue 3 (this is the default configuration). To use default FCoE traffic mapping, in the scheduler map configuration, map the scheduler to the fcoe forwarding class.
How to configure it, you can find it as below.
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/cos-configuring-the-scheduler-buffer-size.html
Hope this answers your query.